1. Field of the Invention
The present invention relates to a piezoelectric actuator comprising a piezoelectric body and a pair of electrodes disposed on either side thereof, and particularly relates to a piezoelectric actuator comprising strontium ruthenate as a bottom electrode.
2. Description of the Related Art
A piezoelectric actuator comprises a piezoelectric element in which a piezoelectric film having an electromechanical transducing function is interposed between two electrodes, the piezoelectric film being constituted by crystallized piezoelectric ceramics. Compound oxides having a perovskite crystalline structure and which can be expressed by the chemical formula ABO3 are known as these piezoelectric ceramics. For example, lead zirconate titanate (PZT) in which lead (Pb) is applied to A and zirconium (Zr) and titanium (Ti) are applied to B is known.
Conventionally, Pt has been used as the electrode material of a PZT-based piezoelectric element. Because Pt has a face-centered cubic lattice structure (FCC) which is a closest packing structure, Pt has strong self-orientation. Therefore, when Pt is deposited on an amorphous material such as SiO2, it becomes strongly oriented in the (111) direction, whereby the orientation property of the piezoelectric film thereon also improves. A problem with such a strong orientation property, however, is that columnar crystal grains grow and that Pb or the like tends to diffuse into the lower layer along the grain boundary. Problems also occur in the adhesiveness of the Pt and SiO2.
Further, when Ti is used in order to improve the adhesiveness of the Pt and SiO2, or TiN or the like is used as a diffusion barrier layer for Pb or the like, the electrode structure becomes complicated. Also, as a result, oxidization of the Ti, diffusion of the Ti into the Pt, and defective crystallization of the PZT occur, and thus the electrical characteristics such as the piezoelectric characteristics deteriorate.
Since such problems exist in Pt electrodes, research is being conducted in the fields of ferroelectric memory and the like into the use of conductive oxides such as RuOx, IrO2 and so on as electrode materials. Among these materials, strontium ruthenate has the same perovskite crystalline structure as PZT and therefore has an excellent bonding property at the interface, eases the epitaxial growth of PZT, and also has an excellent characteristic as a Pb diffusion barrier layer.
However, when strontium ruthenate is used as the bottom electrode of a piezoelectric element, the substance constituting the layer therebelow must have physical properties as a diaphragm and chemical properties to appropriately control the orientation of the strontium ruthenate and secure adhesiveness with the substrate and bottom electrode.
An object of the present invention is to provide a piezoelectric actuator which comprises an optimum layer structure for a case in which (100) orientation strontium ruthenate is used as a bottom electrode. It is a further object to provide a liquid jetting head comprising this piezoelectric actuator.
A piezoelectric actuator according to the present invention comprises a diaphragm comprising (110) or (100) orientation strontium oxide that is formed by means of epitaxial growth on a Si substrate, a bottom electrode comprising (110) orientation strontium ruthenate having a perovskite structure that is formed on the diaphragm; a piezoelectric layer comprising (100) orientation PZT that is formed on the bottom electrode, and a top electrode formed on the piezoelectric layer.
In this piezoelectric actuator, it is desirable that the bottom electrode comprises SrRuO3 layer adjacent to the piezoelectric layer. In this manner, the conductivity as the bottom electrode and the orientation control of the piezoelectric film can be sufficiently secured.
In this piezoelectric actuator, it is desirable for the Si substrate to be oriented in the (100) or (110) direction. In so doing, the strontium oxide can be orientation controlled more favorably.
A liquid jetting head of the present invention comprises the aforementioned piezoelectric actuator, and comprises in the aforementioned Si substrate pressure chambers which are constituted so as to be capable of volumetric change caused by vibration of the diaphragm provided in the piezoelectric actuator.
A liquid jetting device of the present invention comprises the aforementioned liquid jetting head as printing means.